Reductions in Submersed Macrophyte Biomass and Species Richness by the Crayfish Orconectes rusticus

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Abstract
We conducted five 4- to 12-wk in situ enclosure–exclosure experiments in four macrophyte assemblages in three northern Wisconsin lakes to test the impact of grazing by the crayfish Orconectes rusticus. Size of cages and cageless controls ranged from 1 to 3 m2. Adult crayfish reduced total macrophyte biomass in two experiments and reduced species richness in three experiments. The natural occurrence of crayfish in the study lakes ranged from 1 to 223 g live crayfish∙m−2. Crayfish abundance of 19 g∙m−2 reduced total macrophyte biomass 64%, while abundance >140 g∙m−2 eliminated all macrophytes. Decline in species richness and comparison of species abundance over time suggested that crayfish grazed selectively. Both consumptive and nonconsumptive destruction by crayfish caused macrophyte decline. Single-stemmed species were more susceptible to destruction than rosulate or branched species. Our results suggest that crayfish play an important role in structuring the macrophyte and invertebrate communities that they inhabit.